I just bought some Orange cotton for a piece I am working on and BOY am I glad I prewashed ( I dont actually wash, I soak it in hot water for @ 10 min ) then toss it in the dryer. Let's just say the water was VERY colorful!! First piece I have had to bleed in years! It also had a extreme color change when ironing as well.. but as u said, it goes back to normal when it cools.